Process for forming sheet material with excellent surface characteristics

ABSTRACT

A method for making sheet material substantially free from surface imperfections is disclosed, which includes casting the metal into an ingot and subjecting the ingot to a temperature at which its creep resistance is low and to a gas pressure of at least 7,500 psi for a time sufficient to close internal pores and then slicing the ingot into sheets.

United States Patent [191 Salsgiver Feb. 18, 1975 PROCESS FOR FORMING SHEET MATERIAL WITH EXCELLENT SURFACE CHARACTERISTICS [75] Inventor: James A. Salsgiver, Sarver, Pa.

[73] Assignee: Allegheny Ludlum Industries, Inc,,

Pittsburgh, Pa.

[22] Filed: June 28, 1973 [21] Appl. No.: 374,702

[52] US. Cl 29/412, 29/5262, 29/5276,

148/131 [51] Int. Cl B23p 17/00 [58] Field of Search 29/5262, 526.3, 527.5,

[56] References Cited UNITED STATES PATENTS 3,279,005 10/1966 Wehmeyer 164/66 Primary ExaminerC. W. Lanham Assistant Examinen-D. C. Reiley, lll

Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [57] ABSTRACT A method for making sheet material substantially free from surface imperfections is disclosed, which includes casting the metal into an ingot and subjecting the ingot to a temperature at which its creep resistance is low and to a gas pressure of at least 7,500 psi for a time sufficient to close internal pores and then slicing the ingot into sheets.

6 Claims, N0 Drawings PROCESS FOR FORMING SHEET MATERIAL WITH EXCELLENT SURFACE CHARACTERISTICS BACKGROUND OF THE INVENTION Metals typically are ductile and malleable, and they soften upon the application of heat. These typical metal properties have given rise to many methods for forming metal such as hot rolling, cold rolling, forging, extrusion, etc. Metals cast as ingots and then rolled to substantially thinner gauges generally have surface imperfections ironed out during the rolling process. Even if the imperfections are not removed, they are substantially diminished, and if excellent surface characteristics are required for any particular use, surface imperfections can be cut out.

Some alloys remain brittle up to substantially their melting point. These alloys cannot be formed by the usual processes even though some must have excellent surface characteristics to be used for devices that require close tolerances or for other special uses. When an alloy cannot be rolled or forged, it is difficult to form it without surface imperfections, and it is accordingly difficult to provide formed parts of such an alloy within close tolerances.

THE INVENTION This invention provides a process for forming sheets having substantially no surface imperfections from alloys that are too brittle to be worked. The method of this invention includes casting an ingot of the alloy in the usual manner, and after the ingot has solidified, maintaining it under a high pressure and at a temperature that is below a damaging temperature but high enough for the metal to creep and maintaining the ingot at this temperature and pressure for a period of time sufficient to close internal pores. After the porosity of the ingot is reduced substantially to zero, the ingot is removed from the high temperature, high pressure environment and sliced into sheets which may be used as such or have their surfaces polished. The ingot prepared in the manner of this invention will have no internal pores to intersect the surfaces of the slices, and

the slices can therefore be prepared substantially withv no surface defects.

As used in this application, the term ingot will refer to a metal article prepared by solidification of a molten metal or metal mixture. Even when ingots are cast under vacuum, they usually have pores due to contraction of the metal upon solidification and other causes. Although methods are available to cast ingots with no pores or with very few, these methods frequently involve the introduction of impurities or methods of casting where temperature differentials would cause a brittle alloy to crack. The internal pores in ingots are no interconnected and these are the pores that will be dealt with in the process of the present invention. Surface indentations or pores so close to the surface that they are vented to the atmosphere surrounding the ingot cannot be dealt with by the process of this invention and sheet material sliced from an ingot that includes surface pores will usually be discarded.

The term sheet material, as used in this specification, will be used in its usual sense to include any form of metal where the length and width greatly exceeds the thickness. In dealing with unusual alloys for use in small assemblies, sheet material with very small length and width dimensions, such as l in. by l in. and thicknesses of about 0.001 in. are not unusual and are intended to be included within the term sheet material.

The temperature employed in the process of the present invention will depend to large extent upon the alloy being treated. The temperature must be high enough to allow the metal to creep at the applied pressure but not so high as to be damaging to the metal. Damaging temperatures are those that cause detrimental changes in structure, such as causing undesirable grain growth or solubility of phases. In dealing with ferrous metals, temperatures of at least l,7 50F are usually required, and temperatures of about 2,000F are desirable.

The high pressure employed in the process of this invention has no upper limit but must be at least a pressure at which the creeping of the metal to close pores is accelerated so that it may be accomplished in a reasonable time. The pressure may be either gas or liquid pressure but is preferably a gas in that it is difficult to maintain liquid phase at the temperatures required. It is preferred that an inert gas such as argon be employed to avoid detrimental reactions with the metal. Pressures of at least 7,500 psi are usually employed, and pressures of about 10,000 psi or higher are preferred. It is evident that higher pressures are more difficult to achieve, although economics may dictate that investing in equipment suitable for maintaining higher pressures is required to reduce the time of treatment.

From the foregoing, it is obvious that there is a relationship between pressure, temperature and time in effecting the desired closure of pores in the ingot in the process of this invention. When temperature and pressure have been selected, the time required will be that necessary for pore closure to be effected. The required time period can readily be determined for each alloy.

DETAILED DESCRIPTION OF THE INVENTION As described below, the process of the present invention is applied to a special alloy having magnetic properties that make it useful in the construction of magnetic heads to pick up signals from magnetic tapes. These will hereinafter be referred to as tape heads. Tape heads are employed in ordinary recording devices as well as in equipment used in data processing, in video tape manufacture and playing, and for other specialized needs. When tapes are run through a tape head at high speed or where an extremely high fidelity pickup of the recorded signals is required, the demands on the material from which the tape head is made and on the tolerance of its construction are increased.

One suitable alloy for preparing tape heads contains 9-10 percent silicon, 5.56.5 percent aluminum and the balance iron and residuals. Residuals are defined as impurities that do not detrimentally affect the properties of the alloy and which are not deliberately added to the composition.

To be used in a high speed pickup system, a tape head must be capable of being formed to a gap of 50 millionths of an inch. It is apparent that surface indentations in material forming a gap 50 millionths of an inch thick would produce intolerance distortions in the signal picked up by the tape head.

The process of this invention is employed to produce sheet material with substantially no surface imperfections from a metal containing about 9.5 percent silicon, about 6 percent aluminum, and the balance iron and impurities. The alloy is formed as a melt and cast into ingots in the conventional manner. One ingot is placed in an autoclave and heated to 1,900F for 2 hours under 10,000 psi argon pressure; a second ingot is placed in the autoclave heated to 2,100F for 1 hour under 10,000 psi argon pressure; and a third ingot is not treated under pressure at all. The porosity of all ingots was measured, and it was found that the untreated ingot had 8 percent porosity, while both treated ingots had percent porosity.

The treatment at 1,900F for 2 hours and at 2,100F for 1 hour appear to be equivalent in that both treatments resulted in zero porosity and the same magnetic properties of the alloy. The magnetic properties of the alloy were slightly changed by the high pressure, high temperature treatment in the autoclave, although the resultant ingot had gross magnetic properties that were approximately the same as the ingot before such treatment. Some properties were improved, such as the maximum flux density, while other properties were diminished, such as the permeability. However, the gross magnetic properties of the ingot, both before treatment and after treatment, exceeds by a comfortable margin the specifcations of an alloy useful as a tape head.

Table 1 sets forth the properties of the ingots described above.

The ingots, thus prepared, are sliced into sheets 0.005 in. thick and ground and laminated to form tape heads. The surfaces of the sheets cut from ingots prepared in accordance with this invention have substantially perfect contours in that no ingot pores intercept them. The sheets cut from the ingot not prepared in accordance'with this invention have surface imperfections that make them unsuitable for use in forming a take head. Tape heads formed from sheet material made in accordance with this invention result in substantially undistorted pickup of signals from a magnetic tape.

1 claim:

1. A process for forming sheet material substantially ,free from surface imperfections from brittle metal that cannot successfully be rolled comprising:

A. casting an ingot of the brittle metal,

B. maintaining the ingot at a non-damaging temperature at which the metal will creep and under a gas pressure of at least 7,500 psi for a period of time sufficient to substantially eliminate porosity,

C. slicing the resultant non-porous ingot into sheets.

2. The process of claim 1 wherein said pressure is exerted by argon gas.

3. The process of claim 1 wherein the ingot is maintained at temperature and pressure for a period of at least 1 hour.

4. A process for forming sheet material substantially free from surface imperfections comprising:

A. casting an ingot comprising 9-10 percent silicon,

5.5-6.5 percent aluminum and the balance iron and residuals,

B. maintaining the ingot at a non-damaging tempera- 

1. A process for forming sheet material substantially free from surface imperfections from brittle metal that cannot successfully be rolled comprising: A. casting an ingot of the brittle metal, B. maintaining the ingot at a non-damaging temperature at which the metal will creep and under a gas pressure of at least 7,500 psi for a period of time sufficient to substantially eliminate porosity, C. slicing the resultant non-porous ingot into sheets.
 2. The process of claim 1 wherein said pressure is exerted by argon gas.
 3. The process of claim 1 wherein the ingot is maintained at temperature and pressure for a period of at least 1 hour.
 4. A process for forming sheet material substantially free from surface imperfections comprising: A. casting an ingot comprising 9-10 percent silicon, 5.5-6.5 percent aluminum and the balance iron and residuals, B. maintaining the ingot at a non-damaging temperature of at least 1,750*F and a gas pressure of at least 7,500 psi for a period of time sufficient to substantially eliminate porosity, C. slicing the resultant non-porous ingot into sheets.
 5. The process of claim 2 wherein said temperature is at least 1,900*F.
 6. The process of claim 2 wherein said pressure is at least 10, 000 psi. 